1. Field of Invention
The present invention relates to a toner for developing an electrostatic charge image.
2. Discussion of Background
An electrophotographic method usually has steps of forming an electrostatic latent image on a photoconductive photoreceptor by various methods, then visualizing the latent image by means of a toner for developing an electrostatic charge image (hereinafter simply referred to as a “toner”), thereafter transferring the image visualized by the toner to a transfer material such as transfer paper, and fixing the toner image by e.g. heating or pressing. Various methods are known for such steps, and those suitable for the respective processes for forming images are employed.
A pulverization method may be mentioned as one of typical methods for producing toners. This is a method wherein raw materials such as a binder resin, a colorant, a release agent, an electrostatic charge controlling agent, etc. are melt-kneaded, pulverized and classified to obtain toner particles, and it has been widely employed, since it is relatively inexpensive and simple.
In recent years, research and development have been active on a polymerized toner to be produced by a polymerization method such as a suspension polymerization, an emulsion polymerization/coagulation method or a dissolution/suspension method in order to accomplish size reduction or narrower particle size distribution of the toner thereby to accomplish high image quality. With the polymerized toner, the size reduction is relatively easy as compared with the pulverized toner, and a sharp particle size distribution is readily obtainable. Further, the matrix particles may be capsulated, whereby there is a merit such that a toner having heat resistance or low temperature fixing properties can be obtained.
However, the demand for high image quality in the electrophotographic market is particularly strong with respect to a full color image, and various studies are being made to accomplish, in addition to the above-mentioned size reduction of the toner, high durability and control of electrostatic charge/flowability to constantly obtain a high quality.
In order to increase the durability of the toner, a technique of adding spherical silica having a submicron size is known. By this technique, spherical silica present on the outermost surface of the toner particles exhibits a spacer effect, whereby it becomes possible to prevent filming on the photoconductor drum which is problematic during development or to prevent embedding the small size additive in the toner matrix particles. Further, this technique is also effective to improve the transfer efficiency by reducing the adhesion of the toner to the components.
As such spherical silica, silica prepared by a wet method is employed (Patent Documents 1 and 2), but such silica contains a large amount of moisture from its preparation method, whereby the electrostatic property of itself is low, and the electrostatic charge of the toner having such silica added also tends to be low. Accordingly, the toner having such spherical silica added exhibits a certain effect for e.g. improvement of the transfer efficiency or prevention of the OPC (organic photoconductors, hereinafter referred to as “OPC”) filming, but it tends to bring about fogging from the initial stage because of the low electrostatic charge. Such a problem is particularly distinct when printing is carried out under a severer fogging condition i.e. in a high temperature and high humidity environment or by a nonmagnetic one component development system, particularly by means of a high speed machine with a process rate of at least 160 mm/sec. By the technique disclosed in the above documents only, it is not possible to provide an adequate performance to avoid fogging in addition to the OPC filming, etc.
There is a case wherein silica prepared by a dry method is used (Patent Document 3). However, silica prepared by such a conventional method had a small primary particle diameter, whereby the spacer effects were insufficient.
On the other hand, as a means to solve the fogging, a method of adding a large particle diameter additive having an electrostatic property antipolar to the toner, is known. For example, to a negatively chargeable toner, melamine resin particles, etc. may be added. That is, strongly positively chargeable melamine resin particles are attached to or detached from the toner, whereby the toner tends to readily obtain strong and uniform negative chargeability and fogging will be reduced, and there is a further merit such that the electrostatic charge distribution becomes uniform, whereby the uniformity of a solid or half-tone image will be increased.
However, the detached oppositely-charged particles are likely to soil components such as OPC and an electrostatically charged roller, and it is necessary to pay attention to the particle diameter of oppositely-charged particles, adding conditions, etc. Especially when an additive having the same polarity as the toner is used in combination, if its adhesion is weak, it may be peeled off form the toner matrix particles in such a form as surrounded by the oppositely-charged particles and thus tends to increase the soiling of components.
By a study in the present invention, it has been made clear that in a case where such oppositely-charged particles are used in combination with the spherical silica disclosed in the above documents, although the intended improvement to overcome fogging is observed, the electrostatic charges are mutually antipolar, and both of them have readily detachable particle diameters, whereby soiling of components is synergistically increased.
That is, no technique capable of solving the above problems comprehensively has been available.    Patent Document 1: US2002/0115008 A1    Patent Document 2: US2002/0061457 A1    Patent Document 3: JP-A-2001-109185